1. Field of the Invention
The present invention relates to a synchronous rectifier controlled by a current transformer (CT), in particular to a synchronous rectifier with the secondary winding connected by a current transformer in series to switch off the synchronous rectifier switch located on the secondary winding to prevent power loss from crossovers.
2. Description of Related Arts
Nowadays, electronic equipment with high efficiency and low power consumption is in high demand, as consumers begin to pay more attention to the stableness of the power supply and the power consumption. With the advances in the semiconductor manufacturing technology, MOSFET devices are more extensively used to build synchronous rectifying driver ICs. Many types of synchronous rectifying driving ICs are currently available, but their high prices and the complicated control circuits have resulted in the low market acceptance.
A conventional power converter with synchronous rectifying is shown in FIG. 8. according to the structure of the power converter, the secondary winding of the transformer (51) has a comparator (52), and the output of the comparator (52) is used to control the switching of the synchronous rectifying device (53), and the secondary winding of the transformer (51) is also coupled with an induction coil (54), which is connected through a switch (55) to the comparator (52).
In the control circuitry of the above-mentioned power converter, an induction coil (54) is employed to detect any current change through the transformer(51), and then the switching of the synchronous rectifying device (53) is effected directly through the comparator (52) based on the induction signals received. However, the disadvantage of the above circuitry is that the synchronous rectifying device (53) fails to give an early response to the current change in the continuous current output mode; hence, it is subject to the danger of power loss from crossovers.
The main object of the present invention is to provide a synchronous rectifier through the control of a current transformer to prevent power loss from crossovers when operating in the continuous current output mode.
To this end, the architecture of the synchronous rectifier in accordance with the present invention includes a flyback transformer, a switch controller, a current transformer and a driving circuit.
The flyback transformer has a primary winding and a secondary winding. The primary winding is connected to a switching transistor, and the secondary winding is connected to the primary winding of a current transformer.
The switch controller is used to control the switching of a synchronous rectifier switch base on the induction signals received from the secondary winding of the current transformer. One end of the synchronous rectifier switch is connected to the primary winding of the current transformer, and the other end is connected to ground.
If current appears on the secondary winding of the flyback transformer, the secondary winding of the current transformer detects a positive voltage pulse, and the switch controller is enabled to switch on the synchronous rectifier switch. Conversely, if no current is present on the secondary winding of the flyback transformer, the synchronous rectifying switch is turned off.
Furthermore, the current transformer has an auxiliary coil on the primary winding connected to the switching transistor in the switch controller. When the primary winding operating in the continuous current mode detects the cut-off signal, it enables the switching transistor to switch off the synchronous rectifying switch anticipatorily.
The features and structure of the present invention will be more clearly understood when taken in conjunction with the accompanying drawings.